Abstract

The Extensible Authentication Protocol (EAP) is a generic framework supporting multiple types of authentication methods. In systems where EAP is used for authentication, it is desirable to avoid repeating the entire EAP exchange with another authenticator. This document specifies extensions to EAP and the EAP keying hierarchy to support an EAP method-independent protocol for efficient re- authentication between the peer and an EAP re-authentication server through any authenticator. The re-authentication server may be in the home network or in the local network to which the peer is connecting.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

The Extensible Authentication Protocol (EAP) is an authentication framework that supports multiple authentication methods. The primary purpose is network access authentication, and a key-generating method is used when the lower layer wants to enforce access control. The EAP keying hierarchy defines two keys to be derived by all key-generating EAP methods: the Master Session Key (MSK) and the Extended MSK (EMSK). In the most common deployment scenario, an EAP peer and an EAP server authenticate each other through a third party known as the EAP authenticator. The EAP authenticator or an entity controlled by the EAP authenticator enforces access control. After successful authentication, the EAP server transports the MSK to the EAP authenticator; the EAP authenticator and the EAP peer establish Transient Session Keys (TSKs) using the MSK as the authentication key, key derivation key, or a key transport key, and use the TSK for per-packet access enforcement.

When a peer moves from one authenticator to another, it is desirable to avoid a full EAP authentication to support fast handovers. The full EAP exchange with another run of the EAP method can take several round trips and significant time to complete, causing increased handover times. Some EAP methods specify the use of state from the initial authentication to optimize re-authentications by reducing the computational overhead (e.g., EAP Authentication and Key Agreement (EAP-AKA) [RFC4187]), but method-specific re-authentication takes at least 2 round trips with the original EAP server in most cases. It is also important to note that several methods do not offer support for re-authentication.

Key sharing across authenticators is sometimes used as a practical solution to lower handover times. In that case, however, the compromise of one authenticator results in the compromise of key material established via other authenticators. Other solutions for fast re-authentication exist in the literature: for example, see Lopez, et al. [MSKHierarchy]; Clancy, et al. have described the EAP re-authentication problem statement in detail [RFC5169].

In conclusion, to achieve low latency handovers, there is a need for a method-independent re-authentication protocol that completes in less than 2 round trips, preferably with a local server.

This document specifies EAP Re-authentication Extensions (ERXs) for efficient re-authentication using EAP. The protocol that uses these extensions is itself referred to as the EAP Re-authentication Protocol (ERP). It supports EAP method-independent re-authentication

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for a peer that has valid, unexpired key material from a previously performed EAP authentication. The protocol and the key hierarchy required for EAP re-authentication are described in this document.

Note that to support ERP, lower-layer specifications may need to be revised to allow carrying EAP messages that have a code value higher than 4 and to accommodate the peer-initiated nature of ERP. Specifically, the Internet Key Exchange (IKE) protocol [RFC5996] must be updated to carry ERP messages; work is in progress on this project [IKE-EXT-for-ERP].

This document obsoletes RFC 5296 but is fully backward compatible with that document. The changes introduced in this document focus on fixing issues that have surfaced since the publication of the original ERP specification [RFC5296]. An overview of some of the major changes is given below.

o Co-location of the home EAP Re-authentication (ER) and EAP servers is no longer required (see the "ER Server" entry in Section 2).

o The behavior of the authenticator and local ER server during the bootstrapping process has been clarified (Section 5.1); in particular, the authenticator and/or local ER server is now required to check for current possession of the root keys.

o The authenticator is now recommended, rather than just allowed, to initiate the ERP conversation by means of the EAP-Initiate/ Re-auth-Start message (Section 5.3.1.1).

In addition, many editorial changes have been made to improve the clarity of the document and to eliminate perceived ambiguities. A comprehensive list of changes is not given here for practical reasons.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

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This document uses the basic EAP terminology [RFC3748] and EMSK keying hierarchy terminology [RFC5295]. In addition, this document uses the following terms:

ER Authenticator - An entity that supports the authenticator functionality for EAP re-authentication described in this document. All references to "authenticator" in this document imply an ER authenticator, unless specifically noted otherwise.

ER Server - An entity that performs the server portion of ERP described here. This entity may or may not be an EAP server. All references to "server" in this document imply an ER server, unless specifically noted otherwise. An ER server is a logical entity; it may not necessarily be co-located with, or physically part of, a full EAP server.

rMSK - re-authentication MSK. This is a per-authenticator key, derived from the rRK.

keyName-NAI - ERP messages are integrity protected with the rIK or the DS-rIK. The use of rIK or DS-rIK for integrity protection of ERP messages is indicated by the EMSKname [RFC5295]; the protocol, which is ERP; and the realm, which indicates the domain name of the ER server. The EMSKname is copied into the username part of the Network Access Identifier (NAI).

Domain - Refers to a "key management domain" as defined in Salowey, et al. [RFC5295]. For simplicity, it is referred to as "domain" in this document. The terms "home domain" and "local domain" are used to differentiate between the originating key management domain that performs the full EAP exchange with the peer and the local domain to which a peer may be attached at a given time.

ERP allows a peer and server to mutually verify proof of possession of key material from an earlier EAP method run and to establish a security association between the peer and the authenticator. The authenticator acts as a pass-through entity for the re-authentication protocol in a manner similar to that of an EAP authenticator as described in Aboba, et al. [RFC3748]. ERP is a single round-trip exchange between the peer and the server; it is independent of the lower layer and the EAP method used during the full EAP exchange. The ER server may be in the home domain or in the same (visited) domain as the peer and the authenticator (i.e., the local domain).

Figure 1 shows the protocol exchange. The first time the peer attaches to any network, it performs a full EAP exchange (shown in Figure 2) with the EAP server; as a result, an MSK is distributed to the EAP authenticator. The MSK is then used by the authenticator and the peer to establish TSKs as needed. At the time of the initial EAP exchange, the peer and the server also derive an EMSK, which is used to derive an rRK. More precisely, an rRK is derived from the EMSK or from a DSRK, which is itself derived from the EMSK. The rRK is only available to the peer and the ER server and is never handed out to any other entity. Further, an rIK is derived from the rRK; the peer and the ER server use the rIK to provide proof of possession while performing an ERP exchange. The rIK is also never handed out to any entity and is only available to the peer and server.

Two EAP codes -- EAP-Initiate and EAP-Finish -- are specified in this document for the purpose of EAP re-authentication. When the peer identifies a target authenticator that supports EAP re-authentication, it performs an ERP exchange, as shown in Figure 1; the exchange itself may happen when the peer attaches to a new authenticator supporting EAP re-authentication, or prior to attachment. The peer initiates ERP by itself; it may also do so in response to an EAP-Initiate/Re-auth-Start message from the new authenticator. The EAP-Initiate/Re-auth-Start message allows the authenticator to trigger the ERP exchange. The EAP-Finish message also can be used by the authenticator to announce the local domain name.

It is plausible that the authenticator does not know whether the peer supports ERP and whether the peer has performed a full EAP authentication through another authenticator. The authenticator MAY initiate the ERP exchange by sending the EAP-Initiate/Re-auth-Start message and if there is no response MAY send the EAP-Request/Identity message. Note that this avoids having two EAP messages in flight at the same time [RFC3748]. The authenticator may send the EAP-Initiate/Re-auth-Start message and wait for a short, locally configured amount of time. This message indicates to the peer that the authenticator supports ERP. In response to this trigger from the authenticator, the peer can initiate the ERP exchange by sending an EAP-Initiate/Re-auth message. If there is no response from the peer after the necessary number of retransmissions (see Section 6), the authenticator MUST initiate EAP by sending an EAP-Request message, typically the EAP-Request/Identity message. Note that the authenticator may receive an EAP-Initiate/Re-auth message after it has sent an EAP-Request/Identity message. If the authenticator

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supports ERP, it MUST proceed with the ERP exchange. When the EAP-Request/Identity times out, the authenticator MUST NOT close the connection if an ERP exchange is in progress or has already succeeded in establishing a re-authentication MSK.

If the authenticator does not support ERP, it will silently discard EAP-Initiate/Re-auth messages (Section 5.3.2), since the EAP code of those packets is greater than 4 ([RFC3748], Section 4). An ERP- capable peer will exhaust the EAP-Initiate/Re-auth message retransmissions and fall back to EAP authentication by responding to EAP-Request/Identity messages from the authenticator. If the peer does not support ERP or if it does not have unexpired key material from a previous EAP authentication, it drops EAP-Initiate/ Re-auth-Start messages. If there is no response to the EAP-Initiate/ Re-auth-Start message, the authenticator SHALL send an EAP-Request message (typically EAP-Request/Identity) to start EAP authentication. From this point onward, RFC 3748 rules apply. Note that this may introduce some delay in starting EAP. In some lower layers, the delay can be minimized or even avoided by the peer initiating EAP by sending messages such as EAPoL-Start [IEEE_802.1X].

The peer sends an EAP-Initiate/Re-auth message that contains the keyName-NAI to identify the ER server's domain and the rIK used to protect the message, and a sequence number for replay protection. The EAP-Initiate/Re-auth message is integrity protected with the rIK. The authenticator uses the realm in the keyName-NAI field to send the message to the appropriate ER server. The server uses the keyName to look up the rIK. The server, after verifying proof of possession of the rIK and freshness of the message, derives an rMSK from the rRK using the sequence number as an input to the key derivation. The server then updates the expected sequence number to the received sequence number plus one.

In response to the EAP-Initiate/Re-auth message, the server sends an EAP-Finish/Re-auth message; this message is integrity protected with the rIK. The server transports the rMSK along with this message to the authenticator. The rMSK is transported in a manner similar to that of the MSK along with the EAP-Success message in a full EAP exchange. Hoeper, et al. [RFC5749] discuss an additional key distribution protocol that can be used to transport the rRK from an EAP server to one of many different ER servers that share a trust relationship with the EAP server.

The peer MAY request the rMSK lifetime from the server. If so, the ER server sends the rMSK lifetime in the EAP-Finish/Re-auth message.

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In an ERP bootstrap exchange, the peer MAY ask the server for the rRK lifetime. If so, the ER server sends the rRK lifetime in the EAP-Finish/Re-auth message.

The peer verifies the sequence number and the integrity of the message. It then uses the sequence number in the EAP-Finish/Re-auth message to compute the rMSK. The lower-layer security association protocol is ready to be triggered after this point.

The ER server is located either in the home domain or in the visited domain. When the ER server is in the home domain and there is no local ER server in the visited domain, the peer and the server use the rIK and rRK derived from the EMSK; and when the ER server is in the local domain, they use the DS-rIK and DS-rRK corresponding to the local domain. The domain of the ER server is identified by the realm portion of the keyName-NAI in ERP messages.

If the peer is in the home domain or there is no local server in the same domain as the peer, it SHOULD initiate an ERP bootstrap exchange with the home ER server to obtain the domain name.

The defined ER extensions allow executing ERP with an ER server in the home domain. The home ER server may be co-located with a home Authentication, Authorization, and Accounting (AAA) server. ERP with the home ER server is similar to the ERP exchange described in Figure 1.

The defined ER extensions allow the execution of ERP with an ER server in the local domain (access network) if the peer moves out of the home domain and a local ER server is present in the visited domain. The local ER server may be co-located with a local AAA server. The peer may learn about the presence of a local ER server in the network and the local domain name (or ER server name) either via a lower-layer advertisement or by means of an ERP exchange. The peer uses the domain name and the EMSK to compute the DSRK and, from that key, the DS-rRK; the peer also uses the domain name in the realm portion of the keyName-NAI for using ERP in the local domain. Figure 4 shows the ER implicit bootstrapping exchange through a local ER server; Figure 5 shows ERP with a local ER server.

As shown in Figure 4, the local ER server may be present in the path of the full EAP exchange (e.g., this may be one of the AAA entities, such as AAA proxies, in the path between the EAP authenticator and the home EAP server of the peer). In that case, the local ER server requests the DSRK by sending the domain name to the home EAP server by means of a AAA message. In response, the home EAP server computes the DSRK by following the procedure specified in RFC 5295 and sends the DSRK and the key name, EMSKname, to the ER server in the claimed domain (i.e., the local ER server). The local domain is responsible for announcing that same domain name to the peer via a lower layer (for example, through DHCP-based local domain name discovery [RFC6440] or through the EAP-Initiate/Re-auth-Start message with the local ER server).

After receiving the DSRK and the EMSKname, the local ER server computes the DS-rRK and the DS-rIK from the DSRK as defined in Sections 4.1 and 4.3 below. After receiving the domain name, the peer also derives the DSRK, the DS-rRK, and the DS-rIK. These keys are referred to by a keyName-NAI formed as follows: the username part of the NAI is the EMSKname, and the realm portion of the NAI is the domain name. Both parties also maintain a sequence number (initialized to zero) corresponding to the specific keyName-NAI.

If the peer subsequently attaches to an authenticator within the local domain, it may perform an ERP exchange with the local ER server to obtain an rMSK for the new authenticator. ERP with the local ER server is similar to the ERP exchange illustrated in Figure 1.

Each time the peer re-authenticates to the network, the peer and the authenticator establish an rMSK. The rMSK serves the same purposes that an MSK, which is the result of full EAP authentication, serves. To prove possession of the rRK, we specify the derivation of another key, the rIK. These keys are derived from the rRK. Together they constitute the ER key hierarchy.

The rRK is derived from either the EMSK or a DSRK as specified inSection 4.1. For the purpose of rRK derivation, this document specifies derivation of a Usage-Specific Root Key (USRK) or a Domain- Specific USRK (DSUSRK) [RFC5295] for re-authentication. The USRK designated for re-authentication is the rRK. A DSUSRK designated for re-authentication is the DS-rRK available to a local ER server in a particular domain. For simplicity, the keys are referred to without the DS label in the rest of the document. However, the scope of the various keys is limited to just the respective domains for which they are derived, in the case of the domain-specific keys. Based on the ER server with which the peer performs the ERP exchange, it knows the corresponding keys that must be used.

The rRK is used to derive an rIK and rMSKs for one or more authenticators. The figure below shows the key hierarchy with the rRK, rIK, and rMSKs.

rRK | +--------+--------+ | | | rIK rMSK1 ...rMSKn

Figure 6: Re-authentication Key Hierarchy

The derivations in this document are from RFC 5295. Key derivations and field encodings, where unspecified, default to that document.

The rIK is used for integrity protecting the ERP exchange. This serves as the proof of possession of valid key material from a previous full EAP exchange by the peer to the server.

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The rIK is derived as follows:

rIK = KDF (K, S), where

K = rRK and

S = rIK Label | "\0" | cryptosuite | length

The rIK Label is the 8-bit ASCII string:

Re-authentication Integrity Key@ietf.org

The length field refers to the length of the rIK in octets and is encoded as specified in RFC 5295.

The cryptosuite and length of the rIK are part of the input to the KDF to ensure cryptographic separation of keys if different rIKs of different lengths (for example, for use with different Message Authentication Code (MAC) algorithms) are derived from the same rRK. The cryptosuite is encoded as an 8-bit number; see Section 5.3.2 for the cryptosuite specification.

The rIK is referred to by the EMSKname-NAI within the context of ERP messages. The username part of the EMSKname-NAI is the EMSKname; the realm is the domain name of the ER server. In the case of ERP with the home ER server, the peer uses the realm from its original NAI; in the case of a local ER server, the peer uses the domain name received at the lower layer or through an ERP bootstrapping exchange.

An rIK derived from a DS-rRK is referred to as a DS-rIK in the rest of the document. All of the key derivation and properties specified in this section remain the same.

The rIK is the key the possession of which is demonstrated by the peer and the ERP server to the other party. The peer demonstrates possession of the rIK by computing the integrity checksum over the EAP-Initiate/Re-auth message. When the peer uses the rIK for the first time, it can choose the integrity algorithm to use with the rIK. The peer and the server MUST use the same integrity algorithm with a given rIK for all ERP messages protected with that key. The peer and the server store the algorithm information after the first use, and they employ the same algorithm for all subsequent uses of that rIK.

If the server's policy does not allow the use of the cryptosuite selected by the peer, the server SHALL reject the EAP-Initiate/ Re-auth message and SHOULD send a list of acceptable cryptosuites in the EAP-Finish/Re-auth message.

The rIK length may be different from the key length required by an integrity algorithm. In the case of hash-based MAC algorithms, the key is first hashed to the required key length using the HMAC algorithm [RFC2104]. In the case of cipher-based MAC algorithms, if the required key length is less than 32 octets, the rIK is hashed using HMAC-SHA256 and the first k octets of the output are used, where k is the key length required by the algorithm. If the required key length is more than 32 octets, the first k octets of the rIK are used by the cipher-based MAC algorithm.

We identify two types of bootstrapping for ERP: explicit and implicit. In implicit bootstrapping, the ER-capable authenticator or local ER server MUST verify whether it has a valid rMSK or rRK corresponding to the peer. If the ER-capable authenticator or the local ER server has the key material corresponding to the peer, itMUST be able to respond directly in the same way as the home AAA server does without forwarding the DSRK Request to the home domain;

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if not, the ER-capable authenticator or local ER server SHOULD include its domain name in the AAA message encapsulating the first EAP Response message sent by the peer and request the DSRK from the home EAP server during the initial EAP exchange. If such an EAP exchange is successful, the home EAP server sends the DSRK for the specified local AAA client or agent (derived using the EMSK and the domain name as specified in RFC 5295), EMSKname, and DSRK lifetime along with the EAP-Success message. The local AAA client or agentMUST extract the DSRK, EMSKname, and DSRK lifetime (if present) before forwarding the EAP-Success message to the peer. Note that the MSK (also present with the EAP-Success message) is extracted by the EAP authenticator as usual. The peer learns the domain name through the EAP-Initiate/Re-auth-Start message or by means of a lower-layer announcement (for example, DHCP [RFC6440]). When the domain name is available to the peer during or after the full EAP authentication, it attempts to use ERP when it associates with a new authenticator.

If the peer knows there is no local ER server present in the visited domain, it SHOULD initiate ERP explicit bootstrapping (ERP exchange with the bootstrap flag turned on) with the home ER server to obtain the rRK. The peer MAY also initiate bootstrapping to fetch information such as the rRK lifetime from the AAA server.

The following steps describe the ERP explicit bootstrapping process:

o The peer sends the EAP-Initiate/Re-auth message with the bootstrapping flag set (1). The bootstrap message is always sent to the home ER server, and the keyName-NAI attribute in the bootstrap message is constructed as follows: the username portion of the NAI contains the EMSKname, and the realm portion contains the home domain name.

o In addition, the message MUST contain a sequence number for replay protection, a cryptosuite, and an integrity checksum. The cryptosuite indicates the authentication algorithm. The integrity checksum indicates that the message originated at the claimed entity, the peer indicated by the Peer-ID, or the rIKname.

o The peer MAY additionally set the lifetime flag to request the key lifetimes.

o Upon receipt of the EAP-Initiate/Re-auth message from a peer, the ERP-capable authenticator verifies whether it has the local domain name and valid key material corresponding to the peer. If it knows the local domain name and has valid key material corresponding to the peer, it MUST be able to respond directly in the same way as the home ER does, with the local domain name included. If not, it copies the contents of the keyName-NAI into

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the appropriate AAA attribute and may include its domain name in the AAA message encapsulating the EAP-Initiate/Re-auth message sent by the peer.

o Upon receipt of an EAP-Initiate/Re-auth message, the home ER server verifies whether the message is fresh or is a replay by evaluating whether the received sequence number is equal to or greater than the expected sequence number for that rIK. The home ER server then verifies that the cryptosuite used by the peer is acceptable. Next, it verifies the integrity of the message by looking up the rIK and checking the integrity checksum contained in the Authentication Tag field. If any of the checks fail, the home ER server sends an EAP-Finish/Re-auth message with the Result flag set to '1'. Please refer to Section 5.2.2 for details on failure handling. This error MUST NOT have any correlation to any EAP-Success message that may have been received by the EAP authenticator and the peer earlier. If the EAP-Initiate/Re-auth message is well formed and valid, the server prepares the EAP-Finish/Re-auth message. The bootstrap flag MUST be set to indicate that this is a bootstrapping exchange. The message contains the following fields:

* A sequence number for replay protection.

* The same keyName-NAI as in the EAP-Initiate/Re-auth message.

* If the lifetime flag was set in the EAP-Initiate/Re-auth message, the ER server SHOULD include the rRK lifetime and the rMSK lifetime in the EAP-Finish/Re-auth message. The server may have a local policy for the network to maintain and enforce lifetime unilaterally. In such cases, the server need not respond to the peer's request for the lifetime.

* If the bootstrap flag is set, the ER server MUST include the domain name to which the DSRK is being sent along with the EAP-Finish/Re-auth message.

* If the ER server verifies the authorization of a local ER server, it MAY include the Authorization Indication TLV to indicate to the peer that the server that received the DSRK and that is advertising the domain included in the Domain name TLV is authorized.

* An authentication tag MUST be included to prove that the EAP-Finish/Re-auth message originates at a server that possesses the rIK corresponding to the EMSKname-NAI.

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o If the home ER server is involved in the ERP exchange and the ERP exchange is successful, the home ER server SHOULD request the DSRK from the home EAP server; the home EAP server MUST provide the DSRK for the home ER server (derived using the EMSK and the domain name as specified in RFC 5295), EMSKname, and DSRK lifetime for inclusion in the AAA message. The home ER server SHOULD obtain them before sending the EAP-Finish/Re-auth message.

o In addition, the rMSK is sent along with the EAP-Finish/Re-auth message in a AAA attribute (for an example, see Bournelle, et al. [DIAMETER-ERP]).

o The authenticator receives the rMSK.

o When the peer receives an EAP-Finish/Re-auth message with the bootstrap flag set, if a local domain name is present, it MUST use that name to derive the appropriate DSRK, DS-rRK, DS-rIK, and keyName-NAI, and initialize the replay counter for the DS-rIK. If not, the peer SHOULD derive the domain-specific keys using the domain name it learned via the lower layer or from the EAP-Initiate/Re-auth-Start message. If the peer does not know the domain name, it must assume that there is no local ER server available.

o The peer MAY also verify the Authorization Indication TLV.

o The procedures for encapsulating ERP and obtaining relevant keys using Diameter are specified in Bournelle, et al. [DIAMETER-ERP].

Since the ER bootstrapping exchange is typically done immediately following the full EAP exchange, it is feasible that the process is completed through the same entity that served as the EAP authenticator for the full EAP exchange. In this case, the lower layer may already have established TSKs based on the MSK received earlier. The lower layer may then choose to ignore the rMSK that was received with the ER bootstrapping exchange. Alternatively, the lower layer may choose to establish a new TSK using the rMSK. In either case, the authenticator and the peer know which key is used based on whether or not a TSK establishment exchange is initiated. The bootstrapping exchange may also be carried out via a new authenticator, in which case, the rMSK received SHOULD trigger a lower-layer TSK establishment exchange.

When a peer that has an active rRK and rIK associates with a new authenticator that supports ERP, it may perform an ERP exchange with that authenticator. ERP is typically a peer-initiated exchange,

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consisting of an EAP-Initiate/Re-auth and an EAP-Finish/Re-auth message. The ERP exchange may be performed with a local ER server (when one is present) or with the original EAP server.

It is plausible for the network to trigger the EAP re-authentication process, however. An ERP-capable authenticator SHOULD send an EAP-Initiate/Re-auth-Start message to indicate support for ERP. The peer may or may not wait for these messages to arrive to initiate the EAP-Initiate/Re-auth message.

The EAP-Initiate/Re-auth-Start message SHOULD be sent by an ERP- capable authenticator. The authenticator may retransmit it a few times until it receives an EAP-Initiate/Re-auth message in response from the peer. The EAP-Initiate/Re-auth message from the peer may have originated before the peer receives either an EAP-Request/ Identity or an EAP-Initiate/Re-auth-Start message from the authenticator. Hence, the Identifier value in the EAP-Initiate/ Re-auth message is independent of the Identifier value in the EAP-Initiate/Re-auth-Start or EAP-Request/Identity messages.

Operational Considerations at the Peer:

ERP requires that the peer maintain retransmission timers for reliable transport of EAP re-authentication messages. The reliability considerations of Section 4.3 of RFC 3748 apply with the peer as the retransmitting entity.

ERP has the following steps:

o The ERP-capable authenticator sends the EAP-Initiate/Re-auth-Start message to trigger the ERP exchange.

o The peer sends an EAP-Initiate/Re-auth message. At a minimum, the message SHALL include the following fields:

* a 16-bit sequence number for replay protection.

* keyName-NAI as a TLV attribute to identify the rIK used to integrity protect the message.

* cryptosuite to indicate the authentication algorithm used to compute the integrity checksum.

* authentication tag computed over the message.

o When the peer is performing ERP with a local ER server, it MUST use the corresponding DS-rIK it shares with the local ER server. The peer SHOULD set the lifetime flag to request the key lifetimes

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from the server. The peer can use the rRK lifetime to know when to trigger an EAP method exchange and the rMSK lifetime to know when to trigger another ERP exchange.

o The authenticator copies the contents of the value field of the keyName-NAI TLV into an appropriate attribute (e.g., User-Name [RFC2865]) in the AAA message to the ER server.

o The ER server uses the keyName-NAI to look up the rIK. It MUST first verify whether the sequence number is equal to or greater than the expected sequence number. If the ER server supports a sequence number window size greater than 1, it MUST verify whether the sequence number falls within the window and has not been received before. The ER server MUST then verify that the cryptosuite used by the peer is acceptable. The ER server then proceeds to verify the integrity of the message using the rIK, thereby verifying proof of possession of that key by the peer. If any of these verifications fail, the ER server MUST send an EAP-Finish/Re-auth message with the Result flag set to '1' (Failure). Please refer to Section 5.2.2 for details on failure handling. Otherwise, it MUST compute an rMSK from the rRK using the sequence number as the additional input to the key derivation.

o In response to a well-formed EAP-Initiate/Re-auth message, the ER server MUST send an EAP-Finish/Re-auth message with the following contents:

* a 16-bit sequence number for replay protection, which MUST be the same as the received sequence number. The local copy of the sequence number MUST be incremented by 1. If the ER server supports multiple simultaneous ERP exchanges, it MUST instead update the sequence number window.

* keyName-NAI as a TLV attribute to identify the rIK used to integrity protect the message.

* cryptosuite to indicate the authentication algorithm used to compute the integrity checksum.

* authentication tag computed over the message.

* If the lifetime flag was set in the EAP-Initiate/Re-auth message, the ER server SHOULD include the rRK lifetime and the rMSK lifetime.

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o The ER server causes the rMSK along with this message to be transported to the authenticator. The rMSK is transported in a manner similar to the MSK and the EAP-Success message in a regular EAP exchange.

o The peer looks up the sequence number to verify whether it is expecting an EAP-Finish/Re-auth message with that sequence number protected by the keyName-NAI. It then verifies the integrity of the message. If the verifications fail, the peer logs an error and stops the process; otherwise, it proceeds to the next step.

o The peer uses the sequence number to compute the rMSK.

o The lower-layer security association protocol can be triggered at this point.

When a peer is within the range of multiple authenticators, it may choose to run ERP via all of them simultaneously to the same ER server. In that case, it is plausible that the ERP messages may arrive out of order, resulting in the ER server rejecting legitimate EAP-Initiate/Re-auth messages.

To facilitate such operation, an ER server MAY allow multiple simultaneous ERP exchanges by accepting all EAP-Initiate/Re-auth messages with sequence number values within a window of allowed values. Recall that the sequence number allows replay protection. Replay window maintenance mechanisms are a local matter.

If the processing of the EAP-Initiate/Re-auth message results in a failure, the ER server MUST send an EAP-Finish/Re-auth message with the Result flag set to '1'. If the server has a valid rIK for the peer, it MUST integrity protect the EAP-Finish/Re-auth failure message. If the failure is due to an unacceptable cryptosuite, the server SHOULD send a list of acceptable cryptosuites (in a TLV of Type 5) along with the EAP-Finish/Re-auth message. In this case, the server MUST indicate the cryptosuite used to protect the EAP-Finish/ Re-auth message in the Cryptosuite field of that message. The rIK used with the EAP-Finish/Re-auth message in this case MUST be computed as specified in Section 4.3 using the new cryptosuite. If the server does not have a valid rIK for the peer, the EAP-Finish/ Re-auth message indicating a failure will be unauthenticated; the server MAY include a list of acceptable cryptosuites in the message.

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The peer, upon receiving an EAP-Finish/Re-auth message with the Result flag set to '1', MUST verify the sequence number and, if possible, the authentication tag to determine the validity of the message. If the peer supports the cryptosuite, it MUST verify the integrity of the received EAP-Finish/Re-auth message. If the EAP-Finish message contains a TLV of Type 5, the peer SHOULD retry the ERP exchange with a cryptosuite picked from the list included by the server. The peer MUST use the appropriate rIK for the subsequent ERP exchange by computing it with the corresponding cryptosuite, as specified in Section 4.3. If the Pseudo-Random Function (PRF) in the chosen cryptosuite is different from the PRF originally used by the peer, it MUST derive a new DSRK (if required), rRK, and rIK before proceeding with the subsequent ERP exchange.

If the peer cannot verify the integrity of the received message, itMAY choose to retry the ERP exchange with one of the cryptosuites in the list of acceptable cryptosuites (in a TLV of Type 5), after a failure has been clearly determined following the procedure in the next paragraph.

If the replay or integrity checks fail, the failure message may have been sent by an attacker. It may also mean that the server and peer do not support the same cryptosuites; however, the peer cannot determine if that is the case. Hence, the peer SHOULD continue the ERP exchange per the retransmission timers before declaring a failure.

When the peer runs explicit bootstrapping (ERP with the bootstrapping flag on), there may not be a local ER server available to send a DSRK Request and the domain name. In that case, the server cannot send the DSRK and MUST NOT include the Domain name TLV. When the peer receives a response in the bootstrapping exchange without a Domain name TLV, it assumes that there is no local ER server. The home ER server sends an rMSK to the ER authenticator, however, and the peerSHALL run the TSK establishment protocol as usual.

The Identifier field is one octet. The Identifier field MUST be the same if an EAP-Initiate packet is retransmitted due to a timeout while waiting for an EAP-Finish message. Any new (non-retransmission) EAP-Initiate message MUST use a new Identifier field.

The Identifier field of the EAP-Finish message MUST match that of the currently outstanding EAP-Initiate message. A peer or authenticator receiving an EAP-Finish message whose Identifier value does not match that of the currently outstanding EAP-Initiate message MUST silently discard the packet.

In order to avoid confusion between new EAP-Initiate messages and retransmissions, the peer must choose an Identifier value that is different from the previous EAP-Initiate message, especially if that exchange has not finished. It isRECOMMENDED that the authenticator clear EAP Re-auth state after 300 seconds.

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Type

This field indicates that this is an ERP exchange. Two type values are defined in this document for this purpose -- Re-auth-Start (Type 1) and Re-auth (Type 2).

Type-Data

The Type-Data field varies according to the value of the Type field in the re-authentication packet.

Reserved: MUST be zero. Set to zero on transmission and ignored on reception.

One or more Type/Values (TVs) or TLVs are used to convey information to the peer; for instance, the authenticator may send the domain name to the peer.

TVs or TLVs: In the TV payloads, there is a 1-octet type payload and a value with type-specific length. In the TLV payloads, there is a 1-octet type payload and a 1-octet length payload. The length field indicates the length of the value expressed in number of octets.

Domain name: This is a TLV payload. The Type is 4. The domain name is to be used as the realm in an NAI [RFC4282]. The Domain name TLV SHOULD be present in an EAP-Initiate/ Re-auth-Start message.

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In addition, channel binding information MAY be included; seeSection 5.5 for discussion. See Figure 12 for parameter specification.

In order to minimize ERP failure times, the authenticator SHOULD send the EAP-Initiate/Re-auth-Start message to indicate support for ERP to the peer and to initiate ERP if the peer has already performed full EAP authentication and has unexpired key material. The authenticatorSHOULD include the Domain name TLV to allow the peer to learn it without requiring either lower-layer support or the ERP bootstrapping exchange.

The authenticator MAY include channel binding information so that the server can verify whether the authenticator is claiming the same identity to both parties.

The authenticator MAY retransmit the EAP-Initiate/Re-auth-Start message a few times for reliable transport.

The peer SHOULD send the EAP-Initiate/Re-auth message in response to the EAP-Initiate/Re-auth-Start message from the authenticator. If the peer does not recognize the EAP-Initiate code value or if the peer has already sent the EAP-Initiate/Re-auth message to begin the ERP exchange, it MUST silently discard the EAP-Initiate/Re-auth-Start message.

If the EAP-Initiate/Re-auth-Start message contains the domain name, and if the peer does not already have the domain information, the peer SHOULD use the domain name contained in the message to compute the DSRK and use the corresponding DS-rIK to send an EAP-Initiate/ Re-auth message to start an ERP exchange with the local ER server. If there is a local ER server between the peer and the home ER server and the peer has already initiated an ERP exchange with the local ER server, it SHOULD NOT start an ERP exchange with the home ER server.

'B' - The B flag is used as the bootstrapping flag. If the flag is turned on, the message is a bootstrap message.

'L' - The L flag is used to request the key lifetimes from the server.

The remaining 5 bits are set to 0 on transmission and ignored on reception.

SEQ: An unsigned 16-bit sequence number is used for replay protection. The SEQ field is initialized to 0 every time a new rRK is derived. The field is encoded in network byte order.

TVs or TLVs: In the TV payloads, there is a 1-octet type payload and a value with type-specific length. In the TLV payloads, there is a 1-octet type payload and a 1-octet length payload. The length field indicates the length of the value expressed in number of octets.

keyName-NAI: This is carried in a TLV payload. The Type is 1. The NAI is variable in length, not exceeding 253 octets. The EMSKname is in the username part of the NAI and is encoded in hexadecimal values. The EMSKname is 64 bits in

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length, and so the username portion takes up 16 octets. If the rIK is derived from the EMSK, the realm part of the NAI is the home domain name, and if the rIK is derived from a DSRK, the realm part of the NAI is the domain name used in the derivation of the DSRK. The NAI syntax is specified in Aboba, et al. [RFC4282]. Exactly one keyName-NAI attributeSHALL be present in an EAP-Initiate/Re-auth packet.

In addition, channel binding information MAY be included; seeSection 5.5 for discussion. See Figure 12 for parameter specification.

Cryptosuite: This field indicates the integrity algorithm used for ERP. Key lengths and output lengths are either indicated or are obvious from the cryptosuite name. We specify some cryptosuites below:

* 0 RESERVED

* 1 HMAC-SHA256-64

* 2 HMAC-SHA256-128

* 3 HMAC-SHA256-256

HMAC-SHA256-128 is mandatory to implement and SHOULD be enabled in the default configuration.

Authentication Tag: This field contains the integrity checksum over the ERP packet, excluding the Authentication Tag field itself. The length of the field is indicated by the cryptosuite.

'R' - The R flag is used as the Result flag. When set to 0, it indicates success, and when set to '1', it indicates a failure.

'B' - The B flag is used as the bootstrapping flag. If the flag is turned on, the message is a bootstrap message.

'L' - The L flag is used to indicate the presence of the rRK lifetime TLV.

The remaining 5 bits are set to 0 on transmission and ignored on reception.

SEQ: An unsigned 16-bit sequence number is used for replay protection. The SEQ field is initialized to 0 every time a new rRK is derived. The field is encoded in network byte order.

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TVs or TLVs: In the TV payloads, there is a 1-octet type payload and a value with type-specific length. In the TLV payloads, there is a 1-octet type payload and a 1-octet length payload. The length field indicates the length of the value expressed in number of octets.

keyName-NAI: This is carried in a TLV payload. The Type is 1. The NAI is variable in length, not exceeding 253 octets. EMSKname is in the username part of the NAI and is encoded in hexadecimal values. The EMSKname is 64 bits in length, and so the username portion takes up 16 octets. If the rIK is derived from the EMSK, the realm part of the NAI is the home domain name, and if the rIK is derived from a DSRK, the realm part of the NAI is the domain name used in the derivation of the DSRK. The NAI syntax is specified in [RFC4282]. Exactly one instance of the keyName-NAI attribute SHALL be present in an EAP-Finish/Re-auth message.

rRK Lifetime: This is a TV payload. The Type is 2. The value field contains an unsigned 32-bit integer in network byte order representing the lifetime of the rRK in seconds. If the 'L' flag is set, the rRK Lifetime attribute SHOULD be present.

rMSK Lifetime: This is a TV payload. The Type is 3. The value field contains an unsigned 32-bit integer in network byte order representing the lifetime of the rMSK in seconds. If the 'L' flag is set, the rMSK Lifetime attribute SHOULD be present.

Domain name: This is a TLV payload. The Type is 4. The domain name is to be used as the realm in an NAI [RFC4282]. The Domain name attribute MUST be present in an EAP-Finish/ Re-auth message if the bootstrapping flag is set and if the local ER server sent a DSRK Request.

List of cryptosuites: This is a TLV payload. The Type is 5. The value field contains a list of cryptosuites, each of size 1 octet. The cryptosuite values are as specified in Figure 9. The server SHOULD include this attribute if the cryptosuite used in the EAP-Initiate/Re-auth message was not acceptable and the message is being rejected. The serverMAY include this attribute in other cases. The server MAY use this attribute to signal its cryptographic algorithm capabilities to the peer.

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Authorization Indication: This is a TLV payload. The Type is 6. This attribute MAY be included in the EAP-Finish/ Re-auth message when a DSRK is delivered to a local ER server and if the home EAP server can verify the authorization of the local ER server to advertise the domain name included in the domain TLV in the same message. The value field in the TLV contains an authentication tag computed over the entire packet, starting from the first bit of the code field to the last bit of the Cryptosuite field, with the value field of the Authorization Indication TLV filled with all 0s for the computation. The key used for the computation MUST be derived from the EMSK with key label "DSRK Delivery Authorized Key@ietf.org" and optional data containing an ASCII string representing the key management domain for which the DSRK is being derived.

In addition, channel binding information MAY be included: seeSection 5.5 for discussion. See Figure 12 for parameter specification. The server sends this information so that the peer can verify the information seen at the lower layer, if channel binding is to be supported.

Cryptosuite: This field indicates the integrity algorithm and the PRF used for ERP. Key lengths and output lengths are either indicated or are obvious from the cryptosuite name.

Authentication Tag: This field contains the integrity checksum over the ERP packet, excluding the Authentication Tag field itself. The length of the field is indicated by the cryptosuite.

For replay protection, ERP uses sequence numbers. The sequence number is maintained on a per rIK basis and is initialized to zero in both directions. In the first EAP-Initiate/Re-auth message, the peer

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uses a sequence number value of zero or higher. Note that when the sequence number wraps back to zero, the rIK MUST be changed by running a full EAP authentication. The server expects a sequence number of zero or higher. When the server receives an EAP-Initiate/ Re-auth message, it uses the same sequence number in the EAP-Finish/ Re-auth message. The server then sets the expected sequence number to the received sequence number plus 1. The server MUST accept sequence numbers greater than or equal to the expected sequence number.

If the peer sends an EAP-Initiate/Re-auth message but does not receive a response, it retransmits the request (with no changes to the message itself) a preconfigured number of times before giving up. However, it is plausible that the server itself may have responded to the message and the response was lost in transit. Thus, the peerMUST increment the sequence number and use the new sequence number to send subsequent EAP re-authentication messages. The peer SHOULD increment the sequence number by 1; however, it may choose to increment by a larger number. If the sequence number wraps back to zero, the peer MUST run full EAP authentication.

ERP provides a protected facility to carry channel binding (CB) information, according to the guidelines provided by Aboba, et al. (see Section 7.15 of [RFC3748]). The TLV type range of 128-191 is reserved to carry CB information in the EAP-Initiate/ Re-auth and EAP-Finish/Re-auth messages. Called-Station-Id, Calling-Station-Id, NAS-Identifier, NAS-IP-Address, and NAS-IPv6-Address are some examples of channel binding information listed in RFC 3748, and they are assigned values 128-132. Additional values are managed by IANA, based on IETF Review (formerly called "IETF Consensus") [RFC5226].

The authenticator MAY provide CB information to the peer via the EAP-Initiate/Re-auth-Start message. The peer sends the information to the server in the EAP-Initiate/Re-auth message; the server verifies whether the authenticator identity available via AAA attributes is the same as the identity provided to the peer.

If the peer does not include the CB information in the EAP-Initiate/ Re-auth message, and if the local ER server's policy requires channel binding support, it SHALL send the CB attributes for the peer's verification. The peer attempts to verify the CB information if the authenticator has sent the CB parameters, and it proceeds with the lower-layer security association establishment if the attributes match. Otherwise, the peer SHALL NOT proceed with the lower-layer security association establishment.

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6. Lower-Layer Considerations

The authenticator is responsible for retransmission of EAP-Initiate/ Re-auth-Start messages. The authenticator MAY retransmit the message a few times or until it receives an EAP-Initiate/Re-auth message from the peer. The authenticator might not know if the peer supports ERP; in those cases, the peer could be silently discarding the EAP-Initiate/Re-auth-Start packets. Thus, retransmission of these packets should be kept to a minimum. The exact number is up to each lower layer.

The Identifier value in the EAP-Initiate/Re-auth packet is independent of the Identifier value in the EAP-Initiate/Re-auth-Start packet.

The peer is responsible for retransmission of EAP-Initiate/Re-auth messages.

Retransmitted packets MUST be sent with the same Identifier value in order to distinguish them from new packets. By default, where the EAP-Initiate message is sent over an unreliable lower layer, the retransmission timer SHOULD be dynamically estimated. A maximum of 3-5 retransmissions is suggested [RFC3748]. Where the EAP-Initiate message is sent over a reliable lower layer, the retransmission timerSHOULD be set to an infinite value so that retransmissions do not occur at the EAP layer. Please refer to RFC 3748 for additional guidance on setting timers.

The Identifier value in the EAP-Finish/Re-auth packet is the same as the Identifier value in the EAP-Initiate/Re-auth packet.

If an authenticator receives a valid duplicate EAP-Initiate/Re-auth message for which it has already sent an EAP-Finish/Re-auth message, it MUST resend the EAP-Finish/Re-auth message without reprocessing the EAP-Initiate/Re-auth message. To facilitate this, the authenticator SHALL store a copy of the EAP-Finish/Re-auth message for a finite amount of time. The actual value of time is a local matter; this specification recommends a value of 100 milliseconds.

The lower layer may provide facilities for exchanging information between the peer and the authenticator about support for ERP, for the authenticator to send the domain name information and channel binding information to the peer.

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Note that to support ERP, lower-layer specifications may need to be revised. Specifically, RFC 5996 must be updated to include EAP code values higher than 4 in order to use ERP with Internet Key Exchange Protocol version 2 (IKEv2). IKEv2 may also be updated to support peer-initiated ERP for optimized operation. Other lower layers may need similar revisions.

Our analysis indicates that some EAP implementations are not RFC 3748 compliant in that instead of silently dropping EAP packets with code values higher than 4, they may consider it an error. To accommodate such non-compliant EAP implementations, additional guidance has been provided below. Furthermore, it may not be easy to upgrade all the peers in some cases. In such cases, authenticators may be configured to not send EAP-Initiate/Re-auth-Start messages; peers may learn whether an authenticator supports ERP via configuration or from advertisements at the lower layer.

In order to accommodate implementations that are not compliant toRFC 3748, such lower layers SHOULD ensure that both parties support ERP; this is trivial, for instance, when using a lower layer that is known to always support ERP. For lower layers where ERP support is not guaranteed, ERP support may be indicated through signaling (e.g., piggybacked on a beacon) or through negotiation. Alternatively, clients may recognize environments where ERP is available based on preconfiguration. Other similar mechanisms may also be used. When ERP support cannot be verified, lower layers may mandate falling back to full EAP authentication to accommodate EAP implementations that are not compliant to RFC 3748.

This section provides an analysis of the protocol in accordance with the AAA key management guidelines described by Housley & Aboba [RFC4962].

Cryptographic algorithm independence

ERP satisfies this requirement. The algorithm chosen by the peer for the MAC generation is indicated in the EAP-Initiate/ Re-auth message. If the chosen algorithm is unacceptable, the EAP server returns an EAP-Finish/Re-auth message indicating a failure. Algorithm agility for the KDF is specified in Salowey, et al. [RFC5295]. Only when the algorithms used are

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deemed acceptable does the server proceed with the derivation of keys and verification of the proof of possession of relevant key material presented by the peer. A full-blown negotiation of algorithms cannot be provided in a single round-trip protocol. Hence, while the protocol provides algorithm agility, it does not provide true negotiation.

Strong, fresh session keys

ERP results in the derivation of strong, fresh keys that are unique for the given session. An rMSK is always derived on demand when the peer requires a key with a new authenticator. The derivation ensures that the compromise of one rMSK does not result in the compromise of another rMSK at any time.

Limited key scope

The scope of all the keys derived by ERP is well defined. The rRK and rIK are never shared with any entity and always remain on the peer and the server. The rMSK is provided only to the authenticator through which the peer performs the ERP exchange. No other authenticator is authorized to use that rMSK.

Replay detection mechanism

For replay protection of ERP messages, a sequence number associated with the rIK is used. The sequence number is maintained by the peer and the server and is initialized to zero when the rIK is generated. The peer increments the sequence number by one after it sends an ERP message. The server sets the expected sequence number to the received sequence number plus one after verifying the validity of the received message and responds to the message.

Authenticating all parties

ERP provides mutual authentication of the peer and the server. Both parties need to possess the key material that resulted from a previous EAP exchange in order to successfully derive the required keys. Also, both the EAP re-authentication Response and the EAP re-authentication Information messages are integrity protected so that the peer and the server can verify each other. When the ERP exchange is executed with a local ER server, the peer and the local server mutually authenticate each other via that exchange in the same manner. The peer and the authenticator authenticate each other in the secure association protocol executed by the lower layer, just as in the case of a regular EAP exchange.

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Peer and authenticator authorization

The peer and authenticator demonstrate possession of the same key material without disclosing it, as part of the lower-layer secure association protocol. Channel binding with ERP may be used to verify consistency of the identities exchanged, when the identities used in the lower layer differ from those exchanged within the AAA protocol.

Key material confidentiality

The peer and the server derive the keys independently using parameters known to each entity. The AAA server sends the DSRK of a domain to the corresponding local ER server via the AAA protocol. Likewise, the ER server sends the rMSK to the authenticator via the AAA protocol.

Note that compromise of the DSRK results in compromise of all keys derived from it. Moreover, there is no forward secrecy within ERP. Thus, compromise of a DSRK retroactively compromises all ERP keys.

It is RECOMMENDED that the AAA protocol be protected using IPsec or Transport Layer Security (TLS) so that the keys are protected in transit. Note, however, that keys may be exposed to AAA proxies along the way, and compromise of any of those proxies may result in compromise of keys being transported through them.

The home EAP server MUST NOT hand out a given DSRK to a local domain server more than once, unless it can verify that the entity receiving the DSRK after the first time is the same entity that received the DSRK originally. If the home EAP server verifies authorization of a local domain server, it MAY hand out the DSRK to that domain more than once. In this case, the home EAP server includes the Authorization Indication TLV to assure the peer that DSRK delivery is secure.

Confirming cryptosuite selection

Cryptographic algorithms for integrity and key derivation in the context of ERP MAY be the same as that used by the EAP method. In that case, the EAP method is responsible for confirming the cryptosuite selection. Furthermore, the cryptosuite is included in the ERP exchange by the peer and confirmed by the server. The protocol allows the server to reject the cryptosuite selected by the peer and provide alternatives. When a suitable rIK is not available for the

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peer, the alternatives may be sent in an unprotected fashion. The peer is allowed to retry the exchange using one of the allowed cryptosuites. However, in this case, any en route modifications to the list sent by the server will go undetected. If the server does have an rIK available for the peer, the list will be provided in a protected manner and this issue does not apply.

Uniquely named keys

All keys produced within the context of ERP can be referred to uniquely as specified in this document. Also, the key names do not reveal any part of the key material.

Preventing the domino effect

The compromise of one peer does not result in the compromise of key material held by any other peer in the system. Also, the rMSK is meant for a single authenticator and is not shared with any other authenticator. Hence, the compromise of one authenticator does not lead to the compromise of sessions or keys held by any other authenticator in the system, and ERP thereby allows prevention of the domino effect by appropriately defining key scope.

However, if keys are transported using hop-by-hop protection, compromise of a proxy may result in compromise of key material, e.g., the DSRK being sent to a local ER server.

Binding a key to its context

All the keys derived for ERP are bound to the appropriate context using appropriate key labels. The lifetime of a child key is less than or equal to that of its parent key as specified in RFC 4962 [RFC4962]. The key usage, lifetime, and the parties that have access to the keys are specified.

Confidentiality of identity

Deployments where privacy is a concern may find that the use of the rIKname-NAI to route ERP messages serves their privacy requirements. Note that it is plausible to associate multiple runs of ERP messages, since the rIKname is not changed as part of ERP. There was no consensus for that requirement at the time of development of this specification. If the rIKname is not used and the Peer-ID is used instead, the ERP exchange will reveal the Peer-ID over the wire.

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Authorization restriction

All the derived keys are limited in lifetime by that of the parent key or by server policy. Any domain-specific keys are further restricted for use only in the domain for which the keys are derived. All the keys specified in this document are meant for use in ERP only. Other restrictions on the use of session keys may be imposed by the specific lower layer but are out of scope for this specification.

Preventing a DoS attack

A denial-of-service (DoS) attack on the peer may be possible when using the EAP-Initiate/Re-auth message. An attacker may send a bogus EAP-Initiate/Re-auth message, which may be carried by the authenticator in a AAA-Request to the server; in response, the server may send in a AAA reply an EAP-Finish/ Re-auth message indicating failure. Note that such attacks may be possible with the EAPoL-Start capability of IEEE 802.11 and other similar facilities in other link layers and where the peer can initiate EAP authentication. An attacker may use such messages to start an EAP method run, which fails and may result in the server sending a rejection message, thus resulting in the link-layer connections being terminated.

To prevent such DoS attacks, an ERP failure should not result in deletion of any authorization state established by a full EAP exchange. Alternatively, the lower layers and AAA protocols may define mechanisms to allow two link-layer Security Associations (SAs) derived from different EAP key material for the same peer to exist so that smooth migration from the current link-layer SA to the new one is possible during rekey. These mechanisms prevent the link-layer connections from being terminated when a re-authentication procedure fails due to a bogus EAP-Initiate/Re-auth message.

Key material transport

When a DSRK is sent from the home EAP server to a local domain server or when an rMSK is sent from an ER server to an authenticator, in the absence of end-to-end security between the entity that is sending the key and the entity receiving the key, it is plausible for other entities to get access to keys being sent to an ER server in another domain. This mode of key transport is similar to that of MSK transport in the context of EAP authentication. We further observe that ERP is for access authentication and does not support end-to-end data security. In typical implementations, the traffic is in the clear beyond

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the access control enforcement point (the authenticator or an entity delegated by the authenticator for access control enforcement). The model works as long as entities in the middle of the network do not use keys intended for other parties to steal service from an access network. If that is not achievable, key delivery must be protected in an end-to-end manner.

This document is largely based upon RFC 5296; thanks to all who participated in that effort (see Appendix A). In addition, thanks to Yaron Sheffer, Sebastien Decugis, Ralph Droms, Stephen Farrell, Charlie Kaufman, and Yoav Nir for (mostly) useful comments and review.

In writing this document, we benefited from discussing the problem space and the protocol itself with a number of folks including Bernard Aboba, Jari Arkko, Sam Hartman, Russ Housley, Joe Salowey, Jesse Walker, Charles Clancy, Michaela Vanderveen, Kedar Gaonkar, Parag Agashe, Dinesh Dharmaraju, Pasi Eronen, Dan Harkins, Yoshi Ohba, Glen Zorn, Alan DeKok, Katrin Hoeper, and other participants of the HOKEY Working Group. Credit for the idea to use EAP-Initiate/ Re-auth-Start goes to Charles Clancy, and credit for the idea to use multiple link-layer SAs to mitigate DoS attacks goes to Yoshi Ohba. Katrin Hoeper suggested the use of the windowing technique to handle multiple simultaneous ER exchanges. Many thanks to Pasi Eronen for the suggestion to use hexadecimal encoding for the rIKname when sent as part of the keyName-NAI field. Thanks to Bernard Aboba for suggestions in clarifying the EAP lock-step operation, and to Joe Salowey and Glen Zorn for help in specifying AAA transport of ERP messages. Thanks to Sam Hartman for the DSRK Authorization Indication mechanism.